International Patent Applications PCT/AU84/00192 and PCT/AU86/00049 describe methods for backwashing elastic microporous hollow fibre filters. In these filters, a bundle of fibres is contained in a shell or housing and the liquid to be filtered, known as the feedstock, is applied to the outside of the fibres.
During the filtering operation, which may be either to recover clarified liquid or to recover concentrated solids, some of the liquid contained in the feedstock passes through the walls of the fibres and is drawn off from the fibre lumens as filtrate.
Solids contained in the feedstock either pass out of the shell with the remainder of the feedstock carrier stream, or are retained on or in the fibres. These retained solids cause fouling and blockage of the filter.
Industrial practice with the common tube-in-shell microfilters for many years was to apply the feedstock to the inner surface of the fibres by forcing flow through the fibre lumens at such a rate that turbulence scoured the walls of the fibres, retarding blockage by solid material.
In the filter systems described in the above mentioned International Patent Applications, the feedstock is applied to the outer surface of the fibres, with a penalty of low feedstock flow velocity and low turbulence resulting in a rapid rate of blockage of the pores of the fibres.
The blockage of pores is overcome by the application of a backwashing cycle. In the first stage of the cycle, a liquid backwash is applied to the lumens of the fibres such that the liquid passes through the porous walls of the fibres and sweeps retained solids out of substantially all of the pores in the walls of the fibres. In the second stage, a gaseous backwash is applied to the lumens of the fibres such that the gas passes through the larger pores in the walls of the fibres, stretching them and dislodging retained blocking solids.
The application of the backwashing cycle discussed above restores filtrate flux to a high value that is, however, not as high as the initial value. Part of the drop in flux is due to the retention of some solids in the pores of the fibres. With each cycle this slight diminution of the flux reduces the filtration capacity of the fibres. Eventually chemical cleaning is required which is expensive and time consuming.
The need to optimise the frequency of cleaning cycles so as to maximise filtrate flow is discussed in our International Patent Application PCT/AU84/00192, where it is pointed out that it is necessary to maximise the total amount of permeate flow, and minimise the time and permeate lost in a backwash cycle.
In most cases, the filtration process is characterised by a deposition of a layer of solid material on the filter surface that then acts as a filter surface itself. This layer may be constantly changing and is known in the art as a dynamic membrane. In tube-in-shell filters, the constant change in the layer is, in part, caused by the method of application of the feed, which is to apply it in a direction of flow parallel to the fibres, known as cross-flow filtration. The layer is thus potentially being deposited and swept off at the same time.
The dynamic membrane may be unwanted, or it may be beneficial. There are cases where a filter's rejection of solid material is poor until a sufficient layer of dynamic membrane has built up on the filter to give an acceptable product. The dynamic membrane may be composed of solids from the feed, or it may be primarily composed from added filter aid materials.
When a dynamic membrane is desired, care must be taken to ensure that the backwash cleaning cycle is vigorous enough to dislodge blocking solids, but not so vigorous as to sweep away too much of the dynamic membrane. When too much of the dynamic membrane is swept away by the backwash, the filtrate must be diverted back to the feed tank until sufficient dynamic layer is built up again.
In many selective separation cases it has been found that a series of liquid-only backwashes is adequate, with an occassional gaseous backwash. The aim in these cases then, is to maximise the volume of filtrate in a given time, while at the same time ensuring that the quality of the filtrate is maintained.